As it can be seen from the previous expressions, we were not able to match the precise QCD behavior [1]}, [2]}
\(\lim _{q^2\rightarrow \infty }\mathcal {F}_{P\gamma ^\star \gamma ^\star }(q^2,q^2)\propto 1/q^2\) , which will be taken into account as a systematic uncertainty.
This implies an underestimation of the form factor, and thus will enter as an asymmetric error.
However, it is worth to mention that the correct behavior in \(q^2\)  with approximately the correct coefficient is achieved for the singly virtual TFF,
\(\lim _{q^2\rightarrow \infty }\mathcal {F}_{P\gamma ^\star \gamma }(q^2,0)\approx 2F/q^2\) , after the fit to experimental data.
